Powders of an inorganic oxide such as aluminum oxide and zirconium oxide or an inorganic nitride such as aluminum nitride have been known as a raw material for ceramics. These inorganic oxide/nitride powders are molded and then sintered to be ceramics. In general, increasing the density of the polycrystalline ceramics will improve their mechanical strength. This can be achieved using either a method comprising steps of adjusting the particle size of inorganic oxide powders using a dry-way pulverizer and molding and sintering the resulting powders, or a method comprising a step of sintering inorganic oxide powders at a high temperature, and the like. However, the former method has problems such that, in some cases, ceramics with a satisfactorily high density cannot be obtained and that continuous pulverization is difficult to carry out due to accumulation of inorganic oxide powders in a pulverizing chamber. Also, the latter method has problems such that, in some cases, the crystal grains of the resulting polycrystalline ceramics are enlarged, which results in a lowered mechanical strength of the ceramics, and that the production cost increases due to a high energy consumption for the high temperature sintering.